This year’s report covers customer satisfaction with 20 nameplates—both foreign and domestic—which comprise some of the most popular brands bought by U.S. consumers.

After years of improvement, customer satisfaction overall fell 1.2 percent.

“I think that’s simply a consequence of more sales, higher demand, more customers per sales person,” said Fornell.

For American car makers the drop was more pronounced. Five years ago, there was virtually no gap between American and foreign car companies. But, now it appears to be growing.

“U.S. automakers may be stretched too thin, ramping up production to meet rising demand,” says David VanAmburg, ACSI Director. “At more than full capacity, it is not unexpected that quality may be giving way to quantity. This could become problematic once demand slackens, making further sales growth more challenging unless customer satisfaction improves.”

Study founder Claes Fornell notes that we’re talking about vehicles that, historically speaking, have very high quality. But, the rising gap between imports and domestics mean that new buyers who try American brand vehicles, may not come back when they need a new car or truck.

“When you have customers that are not as satisfied as before, chance are they are not going to come back,” he says. “I think that’s where the worry is.”

Luxury brands tend to make their customers happier, and that’s the case in this year’s study.

Honda’s upscale brand Acura enters the the study with an average performance, tied with Ford. Kia and Mazda are flat compared with the prior year, while Buick, BMW, and Hyundai show declines ranging from 4 to 6 percent.

Both GM and Chrysler show mixed results in 2013, with two of three brands suffering downturns in customer satisfaction. For GM, both Buick and Chevrolet drop 6 percent compared to the prior year. The loss for Chevrolet places it in a tie for last with Chrysler’s Dodge, down 2 percent. Chrysler’s Jeep retreats as well, down 4 percent, but the Chrysler nameplate rebounds 6 percent.

The findings of the American Customer Satisfaction Index conflict with studies done by J.D. Power and Consumer Reports, which have shown minimal quality difference between American and import brands.

Fornell defends his study saying that it disregards factors that are irrelevant to quality.

He also says the quality issues are with American brand vehicles are not huge, now. But, says they need to be dealt with in coming years, unless the brands want to lose buyers who are starting to consider their products again.